Conventionally, as a frequency selection filter (below, also referred to as a “filter”) used in an RF (radio frequency) stage of a mobile phone, car phone, or other mobile communication devices, a SAW filter has been widely used. In general, the characteristics required for a frequency selection filter include a broad pass band, low loss, high amount of attenuation, and others. In recent years, particularly for improvement of reception sensitivity and lowering of power consumption in mobile communication devices, there have been rising demands for reduction of loss in SAW filters. Further, in recent years, in mobile communication devices, for reduction of size, antennas have shifted from conventional whip antennas to built-in antennas using dielectric ceramics etc. For this reason, it has become difficult to sufficiently obtain the gain of an antenna. Demands for further reducing the insertion loss of SAW filters have been rising.
Further, in recent years, for reduction of size, lightening of weight, and lowering of costs of mobile communication devices etc., the number of parts used has been increasingly reduced. There have been demands for new functions to be added to SAW filters. One of the demands is for configuration as an unbalanced input-balanced output type or a balanced input-unbalanced output type (type having balance-unbalance conversion function). Here, a “balanced input” or a “balanced output” means that a signal is input or output as a potential difference between two signal lines, and the signals of the signal lines are equal in amplitude, but become inverse in phase. Contrary to this, “unbalanced input” or “unbalanced output” means that a signal is input or output as a potential of one line with respect to a ground potential.
Further, in recent years, along with the rapid changes in mobile communication system, the required specifications on the mobile communication system side have become stricter as well. Namely, a SAW filter with a broader band and having a shape more rectangular in comparison with a conventional filter and having a pass band characteristic such as excellent sharpness has been desired.
In order to realize such a broader band and lower loss of pass band, for example, a double mode SAW resonator filter provided with three IDT (inter digital transducer) electrodes on a piezoelectric substrate and utilizing a vertical first order mode and a vertical third order mode has been proposed. In a double mode SAW resonator filter, particularly, by providing electrode finger (electrode digit) narrow pitch portions at end portions of adjacent IDT electrodes, it is possible to reduce the radiation loss of a bulk wave between IDT electrodes to control the state of the resonance mode and thereby achieve broadening of the band and lowering of the loss (see, for example, Patent Citation 1).
Further, as another means for realizing the required broadening of the band, lowering of the loss, and a balance-unbalance conversion function, a double mode SAW resonator filter forming five IDT electrodes on a piezoelectric substrate has been proposed (see, for example, Patent Citation 2).    Patent Citation 1: Japanese Patent Publication (A) No. 2002-9587    Patent Citation 2: Japanese Patent Publication (A) No. 2005-102328